For the jelly, place the apples, cranberries and cinnamon into a large saucepan and pour over enough water to just cover. Bring to the boil, then reduce the heat and simmer for 10-15 minutes until the apples and cranberries are broken down.
Pass the mixture through a sieve lined with a clean kitchen cloth or muslin into a large jug. Discard the pulp left in the sieve.
For every 600ml/1 pint of liquid you have, add 500g/1lb 2oz of granulated sugar.
Pour the mixture back into a clean pan with a sugar thermometer and cook slowly until it reaches 220C/105F - alternatively, place a spoonful of the jelly onto a cold plate and check if it sets.
Pour the jelly into warm sterilised jars while still hot and allow to cool and set.
For the roast pork, preheat the oven to 180C/350F/Gas 4.
Heat the olive oil in a large frying pan. Season the pork with salt and freshly ground black pepper and place into the frying pan. Cook briefly on all sides until lightly browned all over.
Transfer to a roasting tin and place in the oven for 40-60 minutes, or until the crackling is golden and the pork is completely cooked through.
To serve, carve the pork into portions and place onto serving plates with a spoonful of the cranberry and apple jelly.